In many common types of imaging equipment which employ cut sheet materials, (such as printers, enlargers, and scanners,) there is a need for constraining the sheet (e.g. plain paper, photographic paper, and film) in a flat or otherwise prescribed manner (e.g. cylindrical) such that it may be optically scanned or conversely have an image registered upon its surface. So-called "vacuum platens" have been a common approach to providing an optically unobstructing and physically non-damaging means of constraining sheet materials in this type of equipment. Typically, a vacuum platen or vacuum stage, consists of a rigid (often flat) surface which is either porous or perforated in order to allow a vacuum to be applied to an opposite surface. Atmospheric pressure thereby loads the sheet material against the platen where a combination of pressure and friction forces constrain the sheet both in-plane and transversely.
It is desirable in such apparatus to accommodate different sizes of sheet material. To keep the size of the vacuum pump to a reasonable size, minimize noise and vibration, and minimize power requirements it is known to provide a plurality of plenums under the supporting surface that vary in size corresponding to the size of the sheet material and to provide a valve that can be manually operated to select the desired plenum. See U.S. Pat. No. 3,584,954. It is also known to provide spring loaded flap valves that automatically shut off plenums that are not covered by the sheet material. See U.S. Pat. No. 4,378,155. The flat springs in the flap valves allow a slight oozing of air while closed, thereby creating a hissing noise and requiring the pump to provide continuous flow to compensate for the vacuum lost by the oozing. In automatic apparatus employing sheet material it is also desirable to automatically sense the size of the sheet material in the apparatus. The sheet material holders shown in the above mentioned patents do not provide this capability.